Various methods are known for the extra-corporeal treatment of blood in which the patient's blood flows through a blood treating unit in an extra-corporeal blood circulatory system. One of the chief complications of the extra-corporeal treatment of blood, such as hemodialysis or hemofiltration, is the possibility of air penetrating into the extra-corporeal blood circulatory system. The same risk exists not only with extra-corporeal treatments of blood but also with infusions using infusion solutions.
To separate entrained air bubbles from blood and infusion solutions, known drip chambers are arranged in the venous segment of the extra-corporeal circulatory system and in the infusion line in the respective cases. The known drip chambers are highly reliable in trapping the air bubbles. Nevertheless, there is a risk of air bubbles being infused into the patient intravenously. For a further increase in safety, European Commission Requirement DIN/EN 60601-2-16 sets stringent standards for blood treating apparatus air detectors. The known air detectors are based on the different absorption of ultrasound in liquid and gaseous media and on the scatter of ultrasound at interfaces. As well as the ultrasonic detectors, there are also known air detectors that are based on the different dielectric constants and the different conductivities of liquid and gaseous media. To detect air, signal pulses are coupled into the flowing liquid, and the signal pulses emerging from the flowing liquid are received. It is then determined that air is present when the received signal is below one or more fixed reference levels.
So that the measured results are not falsified, changes in the ambient conditions that affect the received signal have to be compensated for. There are various methods of compensation that are used for this purpose. An arrangement for detecting air bubbles in flowing liquids, which is based on ultrasonic measurement and has compensation for the ambient factors is known from, for example, EP 1182452 A2.
The known methods of monitoring flowing liquids for the presence of air have proved satisfactory in practice for detecting relatively large air bubbles. This is because the relatively large air bubbles, whose volume exceeds approximately 1 μl as individual bubbles and approximately 50 μl as a bolus, cause relatively short and large changes in signal. The ambient factors change relatively slowly by contrast and their effects can therefore easily be detected. In this way, the correction for the ambient factors takes place over a substantially longer period of time than the period occupied by the changes in signal attributable to the air bubbles.
As well as the relatively large individual bubbles, very small bubbles, so-called micro-bubbles, may also occur in hemodialysis treatments. These typically arise when air is able to penetrate into the arterial segment of the extra-corporeal blood circulatory system due to leaks. The individual bubbles, which as a rule are still relatively large at the outset, are first reduced in size by the blood pump. When they then pass through the capillaries of the dialyzer, the inside diameter of which is generally approximately 0.2 mm, they are reduced in size even more. The larger of the small bubbles can be separated out in the venous bubble trap due to their buoyancy whereas the micro-bubbles, which generally have a diameter of approx. 0.2 mm and a volume of approximately 4 μl, are transported out of the bubble trap along with the flow of blood and can travel into the patient. Micro-bubbles having a continuous distribution of diameters and a maximum diameter of approximately 0.3 mm are then present in the patient's bloodstream.
Before a dialysis treatment, the extra-corporeal blood circulatory system is usually flushed out with an isotonic saline solution. In this case, there is a danger that air bubbles which are not flushed free may detach during the treatment and may be infused into the patient undetected in a “cloud” of micro-bubbles.
In the literature (Droste D. W., Kuhne K., Schaefer R. M., Ringelstein B. B; Detection of microemboli in the subclavian vein of patients undergoing haemodialysis and haemodiafiltration using pulsed Doppler ultrasound. Nephrol. Dial. Transplant 2002; 17: 462-466), there are indications that micro-embolisms, whose cause is suspected to be the presence of micro-bubbles, typically occur in dialysis treatments. It is true that the human body will tolerate relatively large amounts of air if it is administered intravenously, because the air can be expired in the lungs or dissolved in the blood. In practice, a continuous rate of air infusion of up to 1.5 ml/min is accepted for a body weight of 50 kg and a maximum blood flow-rate of 600 ml/min. If however these limiting values should be exceeded, then there may be serious complications during the treatment of the blood.